Starch is the major storage carbohydrate in higher plants. The biochemical mechanisms of starch biosynthesis are of interest for understanding fundamental aspects of plant physiology and also for their potential utility in manipulating plant growth for practical purposes. Not only is starch a critical primary source of dietary carbohydrates, but it is also used extensively for various industrial purposes ranging from formation of packaging materials to ethanol production. Despite its wide availability in nature and many industrial applications, the mechanisms by which starch is formed in plant endosperm tissue are not well understood.
Starch consists essentially of a mixture of the homopolysaccharides amylose and amylopectin. Amylose is a linear chain of glucosyl units joined by .alpha.-(1.fwdarw.4) glycosidic bond and normally constitutes about 25% of the total endosperm starch in maize (Zea mays). Amylopectin comprises many linear chains of glucosyl monomers joined by .alpha.-(1.fwdarw.4) linkages and constitutes approximately 75% of the starch. The chains of amylopectin are joined to each other by .alpha.-(1.fwdarw.6) glycosidic bonds, often referred to as branch linkages.
Sugary1 (su1) is one of the oldest known mutations of maize and has been utilized as a sweet corn variety in North America since the 1700s. Phenotypically, immature mutant kernels with the su1 gene mutations accumulate sucrose and other simple sugars, including phytoglycogen (Black et al., 1966; Evensen and Boyer, 1986), which gives corn its desirable sweetness. Specific efforts to improve particular varieties of sweet corn date back to the middle of the nineteenth century. More recently, Sumner and Somers reported in 1944 that the principal polysaccharide storage product in su1 endosperm was a high molecular weight polysaccharide they called phytoglycogen. In 1958, Erlander proposed that phytoglycogen was a normal intermediate in the process of starch synthesis and that a debranching enzyme removed some of the branches by hydrolyzing the .alpha.-1,6 branch points.
Phytoglycogen resembles amylopectin in the respect that .alpha.-(1.fwdarw.4)-linked chains are joined by .alpha.-(1.fwdarw.6) branch linkages, but the ratio of .alpha.-(1.fwdarw.6) to .alpha.- (1.fwdarw.4) linkages is significantly higher in phytoglycogen than it is in amylopectin (Manners, 1985). Although it has been suggested that the su1 gene codes for a starch debranching enzyme (Pan and Nelson, 1984), three different protein isoforms, each with a different level of glycosidase activity, were observed. It is not clear whether this observation was due to differential posttranslational modifications of the proteins, or whether the active enzyme is a multimer which requires combination with products of other gene loci. Further investigation into the mechanisms of starch biosynthesis in plants would be desirable.